Special Interfaces

Abstract

This chapter addresses three topics, each with great practical importance.The wetting of textured surfaces describes not only the wetting of duck feathers but also that of some complex structures the industry is beginning to produce. We have seen in chapter 2 that a standard method of altering the wettability of a surface is to treat it chemically by grafting or adsorbing molecules with wetting characteristics of their own. Another method routinely used by nature is to texture the surface, in other words, to modify the surface roughness. Such textures can be either mildly disordered or strongly disordered and even (to some extent) fractal. If the spatial scale of the roughness is small compared to the wavelength of light the texture does not affect optical transparency. This opens the door to the development of ``super hydrophobic'' windshields, which would combine standard chemical treatments with a surface structure on an ultrafine scale. We will describe the static and dynamical properties of drops that hop, glide, and roll on such surfaces.The wetting of porous media describes the properties of another category of very rough surfaces, albeit with a new twist. The three-dimensional nature of the roughness gives rise to the phenomenon of impregnation. A drop placed on a porous medium does not merely spread on the surface but also penetrates the depth of the support, thereby modifying its wetting properties. A liquid film deposited on a porous substrate recedes via two separate mechanisms: suction and dewetting. This topic encompasses the wetting of powders, rocks and soils, and the paper industry.In a third part dealing with the wetting of soft interfaces, air is replaced by a deformable medium, namely, a rubber. We shall see that the static and dynamical wetting properties are then completely different because the deformation energy of tne elastomer has to be taken into account. The competition between surface and elastic energies introduces a new characteristic length h0, called the elastic length, (which ranges typically in the tens of nanometers). Ordinary capillarity applies only if the spatial scales involved are less than h0. We shall examine the shape of drops and discuss how sandwiched films recede. This phenomenon governs the adhesion of objects on wet substrates (automobile tires, boat glues, contact lenses), as well as certain processes used in the printing industry. In biology, it may play a role in the formation of adhesive contacts between living cells.